1. Field of the Invention
The present invention relates to a manufacturing method of a group III nitride semiconductor crystal and a manufacturing method of a group III nitride semiconductor substrate.
2. Description of the Related Art
Group III nitride semiconductor such as gallium nitride (GaN), indium gallium nitride (InGaN), aluminium gallium nitride (AlGaN) attracts attention as materials for blue light emitting diode (LED) and laser diode (LD). Further, application and development of the group III nitride semiconductor to an element for an electronic device has been started, by utilizing the characteristic of excellent heat resistant property and environment resistant property.
In order to achieve high performance of the above-described device, it is important to reduce crystal defects in epitaxial layers. In recent years, a GaN substrate with high quality is developed by HVPE (Hydride Vapor Phase Epitaxy), which is being spread mainly for the purpose of use for laser diode directed to the next generation DVD.
As a reducing technique of a dislocation density of the GaN substrate, FIELO (Facet Initiated Epitaxial Lateral Overgrowth) is famous (for example, see document 1). In the FIELO, when GaN is grown while forming inclined facets, dislocation is bent by the facets, and therefore propagation of the dislocation in a growth direction can be inhibited. For example, document 2 discloses a dislocation reducing technique in which the principle of the FIELO is thoroughly used. This method of using the FIELO is a technique in which inclined facets are maintained and a thick film is grown on inclined facets, to thereby collect dislocations into a specific part and locally reduce the dislocations in the other part.    (Document 1) A. Usui, H. Sunakawa, A. Sakai and A. A. Yamaguchi: Jpn. J. Appl. Phys., 36 (1997), L899    (Document 2) K. Motoki, T, Okahisa, N. Matsumoto, M. Matsushima, H. Kimura, H. Kasai, K. Takemoto, K. Uematsu, T. Hirano, M. Nakayama, S. Nakahata, M. Ueno, D. Hara, Y. Kumagai, A. Koukitu and H. Seki: Jpn. J. Appl. Phys., 40 (2001), L140
However, the GaN substrate fabricated by the aforementioned conventional method leaves much to be improved. Above all, reduction of a manufacturing cost is a greatest problem to be solved. This is because a base substrate needs to be prepared for each GaN substrate, thus incurring high cost. In order to solve this problem, methods of causing high-rate growth of GaN, multiple wafers growth of GaN, and a bulk method of causing growth of a bulk ingot of thick GaN and cutting-out multiple wafers of GaN from the ingot at once, are examined. Above all, the bulk method is extremely promising, because a substrate having an arbitrary crystal plane excluding C-plane which is available at present, can be manufactured.
However, it is absolutely not easy to grow the ingot of GaN. Particularly, a problem of cracks is serious. When GaN crystal is grown thick, micro cracks are generated for some reason during growth, resulting in an extremely rough surface. GaN that grows on such a rough surface has extremely high defect density, and can not be put into practical use. Such a tendency is remarkable as a growth rate becomes higher. In a case of a relatively slow growth rate of about 100 μm/hour, the problem of the micro cracks is not remarkable so much, and actually, fabrication of the GaN ingot having no-crack, with diameter of 2 inches and thickness of about 5.8 mm, is reported (by Kubo, et. al. 2nd International Symposium on Growth of III-Nitrides (2008), Presentation No. I-Tu-5, “Bulk GaN crystals grown by HVPE”). However, a high growth rate exceeding about 100 μm/hour involves a great problem that cracks are generated. In the growth of a GaN ingot, GaN is grown extremely thick. Therefore, although the high growth rate is favorable from economic viewpoint, such a high-rate thick film growth is greatly problematic.
Further, the reduction of the dislocation density is also an important issue. The dislocation density of the GaN substrates available in the market at present is about 106 cm−2. The dislocation density needs to be further reduced, for further enhancing potential of nitride semiconductor devices. It is known that the dislocation density is reduced as GaN grows thick. The reduction of the dislocations by such a thick film growth of GaN is considered to be caused by attraction between dislocations of Burgers vectors of opposite signs, thereby generating gradual approach between them, and coalescing and vanishing of the dislocations. The reduction rate of the dislocation density by this mechanism becomes extremely slow, with the reduction of the dislocation density. The reason therefore is considered as follows. When the dislocation density is reduced, distance between the dislocations becomes large and the attraction between the dislocations is weakened.
Meanwhile, as another reduction method of the dislocation density, methods according to the aforementioned documents 1, 2 can be given. Although GaN having an area of a low dislocation density of about 104 to 105 cm−2 is obtained, width of the low dislocation density area is about 0.5 mm and is small, between high dislocation density areas. Therefore, there is a problem that manufacture of devices need to be performed, with positions aligned precisely, and there is a limit in a size of the devices that can be manufactured.